1. Field of the Invention
The present invention relates to a light-emitting device including a light-emitting element utilizing an organic electroluminescence (EL) phenomenon (hereinafter also referred to as an organic EL element). The present invention also relates to a substrate used for the light-emitting device.
2. Description of the Related Art
An organic EL element has been actively researched and developed. In a basic structure of an organic EL element, a layer containing a light-emitting organic compound (hereinafter also referred to as a light-emitting layer) is sandwiched between a pair of electrodes. By applying voltage to this element, light emission from the light-emitting organic compound can be obtained.
The organic EL element can be formed into a film; thus, a large-area element can be easily formed. Therefore, the organic EL element has a high utility value as a surface light source that can be applied to lighting or the like.
The organic EL element emits light in a region with a refractive index higher than that of the air; thus, when light is extracted to the air, total reflection occurs in the element or at the interface between the element and the air under a certain condition, which results in a light extraction efficiency of lower than 100%. It is generally said that the light extraction efficiency of the element is approximately 20% to 30%.
Patent Document 1 discloses a structure of an EL device provided with a light extraction layer, in which a lens for refracting light emitted from a light-emitting layer is embedded, between a transparent electrode and a transparent substrate. With such a structure, a reduction in light extraction efficiency due to total reflection at the interface between the transparent electrode and the transparent substrate can be suppressed.
Non-Patent Document 1 discloses a light-emitting device in which an organic EL element is provided over a plastic substrate. A light-emitting device including a plastic substrate can be made light compared to the one including a glass substrate or the like.